The study of MUN provides information about the structure and organization of the human brainstem and spinal cord, and the innervations of muscles. Motor unit number estimation (MUNE) is performed in order to detect and evaluate muscle denervation disorders such as amyotrophic lateral sclerosis (ALS), spinal muscular atrophy (SMA), poliomyelitis and other types of peripheral neuropathy. This MUNE technology has proven to be a useful tool for qualified healthcare professionals to diagnose and assess common neuromuscular disorders. Serial MUNE (multiple measures of MU number over a period of time) has also proven to be valuable in determining the natural time-courses of peripheral neuropathy, and evaluating the effectiveness of intervention procedures.
1. The MUNE Concept
The body's muscles are controlled by motor neurons residing in the spinal cord and brainstem. Motor neurons carry control signals from the brain or spinal cord to muscles. Muscle fiber is the basic functional unit of a muscle, and each motor neuron may innervate one or more muscle fibers through its motor neuron axon. As shown in FIG. 1, motor neuron 1 innervates three muscle fibers 12, 13 and 14 through motor neuron axon 11. Motor neuron 2 innervates two muscle fibers 22 and 23 through motor neuron axon 21.
A motor neuron and all the muscle fibers it innervates are called a motor unit (MU). In FIG. 1, there is shown two MUs: MU1 (made up of motor neuron 1, axon 11 and muscle fibers 12, 13 and 14) and MU2 (made up of motor neuron 2, axon 21 and muscle fibers 22 and 23). A large MU may contain between 50 and 200 muscle fibers. A muscle group may consist of up to hundreds of MUs. An example of a muscle group is the thenar muscles controlling the movement of the thumb.
Neuromuscular diseases may cause muscle fibers to lose the innervation of axons (denervation). If muscle fibers 22 and 23 lose innervation, they are no longer able to contract voluntarily. As a result, the muscle group loses strength and MUN decreases.
Neuromuscular diseases may also cause axon 11 to innervate with muscle fibers 22 and 23 after axon 21 loses its innervation with the two muscle fibers. Although muscle strength may be maintained in this case, the MUN will be reduced. An accurate estimation of MUN will reveal the diseases.
When a motor neuron (e.g., motor neuron 2 in FIG. 1) is activated, an action potential (AP) propagates along the nerve axon (e.g., nerve axon 21 in FIG. 1) and terminal nerve branches (e.g., the terminal nerve branch 25 in FIG. 1) and arrives at the neuromuscular junction or end plate (e.g., end plate 26 in FIG. 1). At the arrival of an AP, the nerve terminals release acetylcholine, which depolarizes the muscle membrane and results in an end plate potential (EPP). When the EPP exceeds the muscle membrane threshold level, it produces a muscle fiber action potential (FAP). The electric field generated in the vicinity of the muscle fibers can be detected by a skin surface electrode located near this field. The combination of the muscle fiber action potentials from all of the muscle fibers of a single motor unit is the single motor unit potential (SMUP). As an example, and looking now at FIG. 1, when an AP 15 activates muscle fibers 12, 13 and 14, the summation of the muscle FAP forms the SMUP_1 16 for motor unit 1.
The electrical activity of multiple MUs of a muscle group can also be measured by the same electrode and is referred to as a compound muscle action potential (CMAP). When all muscle fibers in a muscle group are activated, the measured CMAP is called maximum CMAP. Therefore, the MUN of a muscle group is estimated by using the average size of the SMUPs divided into the maximum CMAP. As an example, and looking now at FIG. 2, SMUP_1, SMUP_2, . . . , and SMUP_N are all single motor unit potentials of a muscle group, and their summation generates the maximal CMAP, i.e., CMAP_max 34. It is not practical to measure every SMUP, but a subset of SMUPs is measurable. The average of the measured SMUP waveforms is considered as a representative SMUP (e.g., SMUP_rep 35 in FIG. 2). Using peak-to-base amplitude as the size of CMAP and SMUP (36 and 37 in FIG. 2), one can estimate the MUN of the muscle group by dividing the numerical value of waveform characteristics 37 into the numerical value of 36. An alternative to using waveform characteristics 37 is to determine the numerical value 38 for each SMUP and then average those numerical values 38 so as to provide an estimate of the representative SMUP size.
2. MUNE Study
Several methods have been developed to estimate MUN for the diagnosis and assessment of neuromuscular disorders. These existing MUNE methods include incremental stimulation (IS), multiple point stimulation (MPS), adapted MPS, F-wave, spike-triggered averaging (STA) and statistical methods. The methods essentially differ from one another in the way in which they acquire the subset of SMUPs. Each of the approaches has specific strengths and limitations.
The IS Method. The IS method estimates SMUP from motor units with a low axon activation threshold. Individual electrodes are first placed over the skin of the subject who is to be studied. The electrode locations are selected based on the operator's knowledge and anatomical landmarks of the subject. Therefore, the electrode size, location, and inter-electrode distance may vary from study to study. Stimulation with intensity just below the activation threshold of motor axons is first applied and null response (baseline) is recorded. The stimulation intensity is then gradually and manually increased until the first recognizable and repeatable muscle response is obtained, representing the activation of the first motor unit. The recorded response is considered the first SMUP. The stimulus intensity is then increased, and a response larger than the previous one is obtained. The difference between the two responses is considered to be the second SMUP, i.e., the net response of the second motor unit. This process is then repeated a number of times. Usually up to 10 discrete increments are obtained, with each increment assumed to represent the addition of one motor unit. The waveforms corresponding to the discrete increments form the subset of SMUP for MUNE.
A physiological phenomenon called alternation complicates utilization of the IS method for MUNE. The alternation phenomenon occurs because activation thresholds of nerve axons often overlap. FIG. 3 illustrates this phenomenon. More particularly, FIG. 3 depicts the probability of motor neuron axon activation as a function of the stimulus strength. It can be seen that motor units MU1 and MU2 have a large common range of activation threshold with varying probabilities. Now consider the following scenario: an initial recorded response is attributed to motor unit MU1, and a subsequent stimulus leads to a response different from the first one. The second response could be the result of both motor units MU1 and MU2, or it could be the result of motor unit MU2 alone. This second condition is sometimes referred to the alternation, as two distinct motor units alternate their activations. If the alternation is not recognized and the difference of the two responses is considered as the SMUP of MU2, the response of MU2 will be under-estimated. Consequently, the MUN will be over-estimated. Indeed, the IS method often results in over-estimating the MUN in comparison with other MUNE methods.
The IS method is carried out manually by an expert. Recognizing the potential pitfall of alternation, the expert often spends a significant amount of time to determine whether the changes observed in the most recent response are due to alternation or activation of a new motor unit. If alternation involves n motor units, a maximum of (2n−1) alternation waveform patterns exist. For n>2 (e.g., motor units 3, 4, and 5 in FIG. 3), determination of the alternation pattern can prove to be a challenge too great for any human operator. Even without the complication of alternation, it is possible that electronic and physiological noise could also be mistaken as new motor units.
Multi-Point Stimulation (MPS). To sidestep the complexity of sorting out motor unit alternation patterns, the multi-point stimulation (MPS) method was developed. With this approach, multiple stimulation sites along the nerve axon path are used. For each stimulation site, a stimulus with an intensity low enough to activate only one motor unit is used and the response is recorded as SMUP. The stimulating electrode is then moved slightly along the axon path and the process is repeated. Often the responses are thereafter manually examined, and only those responses with different morphology and amplitude are accepted. This is to ensure that distinct motor units are sampled. Because each SMUP is obtained with a different latency (i.e., with a different nerve impulse traveling time from stimulator to detector), the SMUPs should not be combined directly to obtain a representative SMUP for determining the size of average SMUP. Instead, the feature of individual SMUP (e.g., amplitude, negative peak area, etc.) has to be calculated first and then averaged with other individual SMUPs. However, the SMUP feature obtained using this approach is generally inferior to the feature obtained from the averaged SMUP. This is because the feature may not be additive (i.e., the amplitude of two SMUPs added together is smaller than the summation of the two SMUP amplitudes, unless the peaks are aligned perfectly in time). This phenomenon may lead to under-estimation of the MUN.
Adapted MPS. The adapted MPS method is a hybrid of the IS and MPS methods. It still utilizes multiple stimulation sites. However, the method attempts to elicit more than one SMUP until alternation becomes evident. Often this means that ˜2-3 SMUPs can be obtained at each stimulation site. While this method reduces the number of stimulation sites, it still shares the limitation of the MPS method, i.e., that SMUPs evoked from different stimulation sites cannot be easily combined. Both the Adapted MPS and MPS approaches can only be applied to long, accessible nerves.
The F-Wave Approach. Another method using surface electrode stimulation is the F-Wave approach. With this approach, the electrode placement is similar to that in the IS method. F-wave is a late response of a muscle group. It is generated by antidromic (“backfiring”) of motor neurons following stimulation of peripheral nerve axons. Only a small fraction (i.e., about 2-5%) of motor neurons will backfire for each stimulus, with the backfiring occurring on a randomized basis (i.e., different motor neurons will backfire even under identical stimulation conditions). After the maximum CMAP is acquired, its size is determined. A commonly-used size measure is peak-to-base amplitude (shown at 36 in FIG. 2), peak-to-peak amplitude (shown at 42 in FIG. 2), and area-under-the-peak (shown at 41 in FIG. 2). The stimulus intensity is adjusted to evoke a CMAP with a size of 10-50% of the maximum. At the reduced intensity, the axons are stimulated up to 300 times at the same stimulation location. F-waves with identical morphology and latency are considered as the same SMUP because the probability for multiple motor neurons to backfire together more than once to form the same morphology is much lower. For illustration, it is assumed that the backfiring probability for each motor neuron is p=0.02. Given that an F-wave morphology is observed, the probability of the same F-wave morphology being observed again is p=0.02 if the F-wave is from a single motor neuron. The probability becomes p*p=0.0004 if the F-wave is a result of two motor neurons' backfiring together. The F-Wave method does not involve the challenge of assessing alternation, but the identified SMUPs may have different latency. Therefore, combining them directly may not yield a representative SMUP. However, the key assumption that an F-wave is a single motor unit potential if it occurs more than once may not be valid, particularly in a diseased population. Some motor neurons are also more likely to backfire than others, making the observed SMUPs a biased sample of the total motor unit pool.
Spike-Triggered Averaging (STA). The STA method uses surface electrodes to record SMUPs either with voluntary muscle contraction by subjects or by applying electric stimulation. With the voluntary muscle contraction approach, a needle is inserted into muscle to detect the spike associated with a specific SMUP. The spike is then used as the trigger to synchronize the surface recordings in order to estimate the corresponding SMUP. The needle position is adjusted multiple times to probe different motor units. With the electric stimulation approach, a needle is inserted to deliver micro-stimulation to individual motor axons. While kept at the same insertion point, the needle electrode tip position is manually adjustable to activate different axons in the nerve bundle. The surface recorded responses are time-synchronized with stimulus onset and averaged to yield SMUP. Multiple SMUPs acquired with either approach are utilized in the same manner as with other methods to generate MUNE. One advantage of the STA technique is that it can reach proximal muscles, whose nerves are usually inaccessible by other methods. However, this approach is invasive, causes greater discomfort to subjects than other methods, and requires skillful operators.
Statistical Methods. Statistical methods are fundamentally different from other MUNE methods. With these statistical methods, a series of CMAPs are recorded at fixed, sub-maximal stimulation intensities. Axons near threshold will be activated intermittently, resulting in small CMAP variations. The estimated size of individual SMUPs that are activated intermittently is calculated from the variance of the CMAP. The total MUNE is calculated by dividing the maximal CMAP size by the estimated SMUP size obtained at different intensities. This MUNE method evaluates motor unit size throughout the entire response range from threshold to supramaximal. It depends less on operator expertise and eliminates the issue of alternation, but requires a high number of stimuli.
Thus there is a need for a new approach for estimating motor unit number (MUN) which addresses the problems of the prior art.